Trine 2, it looks like Frozenbyte has sweetened up the deal. The game is now available for pre-purchase on Steam for $15, and those who take the plunge will receive access to the multiplayer beta. The beta will include two levels, though few other details have been revealed, including when the beta will actually start. Given the game is set to be released this December, it shouldn't be too long.
Update: Turns out the Trine 2 beta is available right now! Go grab it off of Steam if you already pre-ordered.